1. Field of the Invention
The present invention relates to an optical recording medium and a method for producing the optical recording medium. More particularly, the invention relates to an optical recording medium provided with a plurality of recording layers on which information can be recorded holographically are laminated, and a method for producing the optical recording medium by spin-coating.
2. Description of the Related Art
Conventionally, rewritable optical disk recording media, such as 12 cm-diameter DVD-RAM capable of high-density, double-sided recording of 5.2 GByte (Gigabyte) with a phase change system, have already been widely used. However, in these optical disk recording media, since data are recorded in a plane, high-density recording is restricted by diffraction limit of radiation and has come close to a physical limitation. In order to achieve a higher recording capacity, a three-dimensional (volume) recording system including a depth direction is required. With respect to the above-mentioned volume-type recording medium, a volume holography memory capable of volume-recording hologram lattices has been regarded as a prospective medium.
Here, in order to achieve a large capacity in the volume holography memory, “a thicker recording layer” is the most important factor. In general, the thicker the hologram, the narrower the incident angle conditions for allowing diffraction, and only a slight offset from Bragg conditions causes diffraction radiation to disappear. The multiple angle method in the volume holography memory utilizes this angle selectivity. In other words, a plurality of holograms are formed in the same volume, and by controlling the incident angle of reading radiation, it becomes possible to read a desired hologram without crosstalk. In this manner, by increasing the film thickness of the recording layer so as to improve the angle selectivity, the multiplicity can be increased so that the recording capacity is increased.
In DVDs and the like, in general, the spin-coating has been used for forming a recording layer. In the spin-coating, a coating solution is dropped onto portions near center on a rotating disk, the coating solution, driven by centrifugal force, flows and reaches the periphery of the disk to form a film, excessive coating solution flies out from the periphery of the disk, then the solvent is removed from the film. For example, recording layers and non-recording layers are laminated alternately by using the spin-coating to form a multi-layer holographic memory (see Japanese Patent Application Laid-Open (JP-A) No. 11-250496). In this example, a solution A prepared by dissolving polymer containing a photo-reactive component in pyridine is spin-coated to form a recording layer having a thickness of 1 μm, and on this recording layer, a solution B prepared by dissolving polyvinyl alcohol in water is spin-coated to form a non-recording layer having a thickness of 8 μm.
However, it is very difficult to form a recording layer having a thickness of several tens μm or more to be used for the volume holography through single spin-coating process. Since the holography memory disclosed in JP-A No. 11-250496 is not used for multiplex recording, the thickness of the recording layer formed in the method disclosed in JP-A No. 11-250496 is as thin as approximately 1 μm. Further, since holograms recorded in the laminated recording medium are formed in the respective recording layers for avoiding crosstalks, this method is not suitable for the volume holography.